High power lasers with mutually coherent multiple output beams allow generation of larger total optical power levels than are possible with single-output-beam devices because of limitations such as optical power loading of mirrors and attenuation in the gain medium. Mutually coherent optical beams offer extremely high directionality of pointing by controlled coherent combination of several beams, similar to phased array usage of multiple antennas or horns in radio frequency or microwave devices. It is desirable to have output beams which are collimated and phase related, since otherwise separate optical systems for achieving these goals are likely to be required.
Optical power is lost in standing wave resonators when the gain medium is too long. The optical power is removed at the end. The gain is saturated therefore the optical power increases linearly with length, but the attenuation depletes the power in proportion to its intensity. Therefore there is a maximum length one can build efficient standing wave resonators. As the length is increased beyond the critical length, the optical power generated by the gain is exactly matched by that attenuated. This places a limit on the in phase maximum power that can be produced by a single aperture. If this limit is to be exceeded, then other methods must be found.
One method of overcoming this limit to obtain higher levels of total output flux can be achieved by simultaneous operation of independent oscillators, but this will not yield mutual coherence as needed for high directionality. Mutual coherence of multiple output beams can be achieved by using a beam splitter to obtain multiple output beams from a single flux generator (power oscillator) if total power output is low, however, flux loading of beamsplitters becomes intolerable for high flux levels. The combination of high flux and mutually coherent multiple beams can be achieved by frequency locking each of several individual power oscillators by injection into each of them of a low-power signal. The various low-power injection signals can be made to have the necessary mutual coherence by obtaining each of them by beam-splitting of a common reference signal. Unfortunately, this frequency locking method presents great practical difficulties. The laser system must be rigidly controlled to extremely tight specifications regarding such features as alignment of the input signal with the optic axis of each resonator and control of mirror separation for each of the resonators to match one another. These problems and other attendant difficulties are overcome by the subject resonator which automatically provides for multiple outputs from a single overall optical resonator path having all sections of gain path length less than the attenuation critical gain path length.